A witness may give expert, scientific, or specialized opinion only if the witness is qualified as as an expert.

United States v. Bakhtiar, 994 F.2d 970, 977 (2d Cir. 1993) (upholding trial court's admission of co-conspirator's statements as statements against penal interest because (1) "[co-conspirator] McDonald was unavailable....Federal Rule of Evidence of Evidence 804(b)(3) or a comparable State rule creates a hearsay exception for a statement which was at the time of its making so far contrary to the declarant's pecuniary or proprietary interest, or so far tended to subject the declarant to civil or criminal liability, or to render invalid a claim by the declarant against another, that a reasonable person in the declarant's position would not have made the statement unless believing it to be true. A statement tending to expose the declarant to criminal liability and offered to exculpate the accused is not admissible unless corroborating circumstances clearly indicate the trustworthiness of the statement.

Cf. Koch v. Cox, 489 F.3d 384, 388-391 (D.C. Cir. 2007) (court assumes that claimant-patient waives psychotherapist-patient privilege by putting patient's mental condition in issue and considers several possible tests for determing when mental condition has been put in issue; "we hold that a...The psychotherapist-patient or therapist-patient privilege may protect communications between claimant and psychotherapist or therapist even if the claimant's claim rests on claimant's mental or emotional condition.

United States v. Brown, 327 F.3d 867, 872 (9th Cir. 2003) (reversing conviction for wire fraud because district judge failed to give appropriate curative instructions after prosecutor, in closing argument, seeking to show defendant's criminal propensity, referred to evidence of defendant's other...A person's character, or propensity, is inadmissible to show that person's conduct on a specific occasion.